cell biology

very differently. they are both sweet mechanisms.

lac operon

this is controlled by both activation and inhibition. if there is glucose in the cell then the activator is inhibited. when there is high levels of AMP compared to ATP the inhibitor is inhibited. therefore only in the absence of glucose (the major source of energy) and ATP will the cell use lactose instead.

trp operon

this one has a length of mRNA that controls the translation of the proteins. there are 3 sections that can form a hairpin structure on this initial sequence. there is 3 codons at the start that code for tryptophan. this means that 3 tryptophan have to be near by to be used. if these 3 are around then the ribosome goes quickly through it and the hairpin structures that can form make a termination sequence. this stops the translation of the mRNA. if there is no tryptophan around the ribosome is stalled and therefore other hairpin structures are formed stopping the termination sequence from being formed. this therefore means the ribosome can make the proteins that are needed for the production of tryptophan

Repressible genes are those in which the presence of a substance (a co-repressor) in the environment turns off the expression of those genes (structural genes) involved in the metabolism of that substance.

e.g., Tryptophan represses the expression of the trp genes.


An inducible gene is a gene that is expressed in the presence of a substance (an inducer) in the environment. This substance can control the expression of one or more genes (structural genes) involved in the metabolism of that substance.

eg , lactose induces the expression of the lac genes that are involved in lactose metabolism. An certain antibiotic may induce the expression of a gene that leads to resistance to that antibiotic.


As with all operons, the trp operon consists of the repressor, promoter, operator and the structural genes. In this system, though, unlike the lac operon, the gene for the repressor is not adjacent to the promoter, but rather is located in another part of the E. coli genome. Another difference is that the operator resides entirely within the promoter

The trp operon is a repressible system. The primary difference between repressible and inducible systems is the result that occurs when the effector molecule binds to the repressor. With inducible systems, the binding of the effector molecule to the repressor greatly reduces the affinity of the repressor for the operator, the repressor is released and transcription proceeds. The lac operon is an example of an inducible system. With repressible systems, the binding of the effector molecule to the repressor greatly increases the affinity of repressor for the operator and the repressor binds and stops transcription. Thus, for the trp operon , the addition of tryptophan (the effector molecule) to the E. coli environment shuts off the system because the repressors binds at the operator.

Inducible:
An inducible gene is a gene that is expressed in the presence of a substance (an inducer) in the environment. This substance can control the expression of one or more genes (structural genes) involved in the metabolism of that substance. For example, lactose induces the expression of the lac genes that are involved in lactose metabolism. An certain antibiotic may induce the expression of a gene that leads to resistance to that antibiotic.
Induction is common in metabolic pathways that result in the catabolism of a substance and the inducer is normally the substrate for the pathway.

Repressible:
Repressible genes are those in which the presence of a substance (a co-repressor) in the environment turns off the expression of those genes (structural genes) involved in the metabolism of that substance.

e.g., Tryptophan represses the expression of the trp genes.

Repression is common in metabolic pathways that result in the biosynthesis of a substance and the co-repressor is normally the end product of the pathway being regulated.

The primary difference between repressible(Trp) and inducible system(Lac) is the result that occurs when the effector molecule binds to the repressor. With inducible systems, the binding of the effector molecule to the repressor greatly reduces the affinity of the repressor for the operator, the repressor is released and transcription proceeds. The lac operon is an example of an inducible system( induced in the presence of lactose). With repressible systems, the binding of the effector molecule to the repressor greatly increases the affinity of repressor for the operator and the repressor binds and stops transcription. Thus, for the trp operon , the addition of tryptophan (the effector molecule) to the E. coli environment shuts off the system because the repressors binds at the operator.

first off i want to remind you that operons exist only in bacteria...i got a question wrong on a test because i didnt make that clear enough to myself


ok back to the answer.

there are two kinds of operons, ones that are inducible (meaning that they are naturally in the "off" position) and repressible (meaning they are natually in the "on" position).

a lac operon is inducible, and a trp operon is repressible.

inducible operons have an operator that prevents the DNA from being read and therefore translated. When the appropriate chemical, like lactose for the lac operon comes into contact with a certain protein, it will alter the protein allosterically so that it is now able to fit into an active site in the operator of the lac operon. once this operator is bound with the protien, it allows the DNA of the lac operon to be read. when this happens, the proteins that are responsible for digesting the lactose in the system of the bacteria are produced, and the lactose dissipates. when this happens there is not enough to keep the protein bound to the operator, and the protein disconnects, therefore turning off the operator.

the trp operon works in a similar way, the only difference being that it is always turned on, and when trp binds to the protein, the protein then alters the operator so that DNA cannot be read. when the amount of trp in the system diminishes, the operator turns back on again.

1. in lac operon; gene activity is induced when lactose is present in the medium, whereas in case of trp operon, repression of the gene activity takes place in presence of tryptophan in the medium.

2. lac operon spans about 4-6kb...whereas trp operon spans abut 7kb.

3. lac operon helps in the breakdown of lactose into glucose and galactose, to generate energy( catabolic pathway ) in case of trp operon, it helps in the synthesis of enzymes required for the formation of the amino acid Tryptophan( anabolic pathway ).

4. lac operon is an example of positive regulation ; and trp operon is an example of negative regulation.


Trp operon is also regulated by other mechanism called attenuation while no such even occurs in lac operon